The present invention relates to a receiver apparatus or equipment for a radio transmission system. More specifically, the present invention is concerned with a radio receiver apparatus for receiving discriminatively intrinsic data or information signals which are transmitted through radio communication between or among office automation apparatuses or machines.
Heretofore, for transfer or communication of information or data among a plurality of office automation (OA) machines, there has been adopted a method of interconnecting the machines to one another through the medium of cables. However, for interconnecting a large number of such machines by cables, troublesome and expensive wiring works are required, which provides difficulty to rearrangement or alteration of the machines upon renewal of office layout as well as installation of additional machines. For solving such problems, there is conceivable a radio or wireless interconnection or communication among the machines by making use of electromagnetic wave or radio channels. However, such wireless communication facilities are not adopted widely because of problems such as limitation imposed to the radio output power as well as carrier frequency, interference with other machines and so forth.
On the other hand, as a communication system designed for specific application, there has been developed a so-called spread spectrum communication system according to which data or information signal is spread over a wide frequency band so as to allow the signal to be transmitted with electromagnetic wave of low power density. This system or scheme is advantageous in that disturbance to the other machines can be suppressed to a minimum and that the system is less susceptible to interference of the other machines because of spreading and demodulation based on a pseudo noise coding or sequentialization. In the spread spectrum communication system, there is adopted a spreading code or sequence generally known as PN code or sequence for modulating the information or data signal over a wide band. However, in the transmitter/receiver system in which the spread spectrum scheme is adopted, the correlative demodulation circuit for demodulating the signal spread by using the PN code or sequence is extremely complicated in respect to the circuit configuration. Under the circumstances, several demodulating schemes have been proposed. Among them, there may be mentioned a scheme or method of employing a SAW (Surface Acoustic Wave) matched filter for the correlative demodulation or detection, as a method capable of demodulating the signal with high reproducibility with a simple circuit configuration, as is disclosed in JP-A-5-327661.
The scheme for reproducing data by detecting correlated data with a delay by using the SAW matched filter as the correlator is certainly advantageous in that the time taken for establishing synchronization can be shortened and that the circuit structure can be simplified without need for PN generating circuit, synchronizing circuit and the like for the demodulation. However, because of the radio communication, the output level after the detection may change finely at random, giving rise to a problem that identification error due to deterioration of the S/N ratio is likely to take place even when the signal level inputted to the demodulator is stabilized by resorting to the AGC (Automatic Gain Control) technique.
In the light of the state of the art described above, it is an object of the present invention to provide an improved structure of the radio receiving apparatus which can ensure improvement of S/N ratio of the demodulated signal by taking advantage of the characteristics of the SAW matched filter mentioned above.
In view of the above and other objects which will become apparent as the description proceeds, there is provided according to a first general aspect of the invention a first scheme according to which the demodulated output of the SAW matched filter is subjected to a level detection to be thereby used as a reference signal for discriminative identification of the received signal with a delay after detection thereof.
Further provided according to a second general aspect of the invention is a scheme for generating a signal synchronized with a burst-like demodulated signal of the SAW matched filter from a data synchronizing pulse signal derived from the correlative demodulation, whereon the aforementioned signal is superposed onto the reference signal generated according to the first scheme mentioned above.
By detecting the level of the demodulated signal outputted from the SAW matched filter and using it as the reference signal for a comparator which is provided for the signal discrimination after the detection with a delay, it is possible to change or vary the reference signal employed for the discriminative identification of the signal by following minute or fine change of detection output level due to variation of the received signal level, whereby S/N ratio can be protected against deterioration or degradation due to the level variation.
Furthermore, by generating the signal synchronizing with the burst-like demodulated signal of the SAW matched filter from the synchronizing pulse of the data generated through the correlative demodulation and superposing the synchronous signal onto the reference signal mentioned above, the S/N ratio can significantly be improved by broadening the dynamic range with the reference signal being set to a low level at the time point or timing for the signal discrimination or identification, while the level of the reference signal is increased during the other period, to thereby suppress erroneous operation due to noise to a possible minimum.
Thus, according to the teachings of the present invention, the burst-like signal can discriminatively be identified or detected reliably and stably with a simple structure in a system where the signal level varies constantly such as the radio communication system, whereby a radio receiving apparatus ensuring an improvement of the S/N ratio of the received signal.